16

How to Fix It

1. Drop the set temperature.

   Use the controller to set the temperature to 120°F for testing.
   If the unit runs without code 16 at 120°F but trips at 140°F, the heat exchanger and sensor are likely fine — the system was just being asked to push water hotter than it can safely deliver.

2. Descale the heat exchanger.

   Hook a descaling pump to the service valves.
   Circulate a 50/50 vinegar and water mix for 60 minutes, then a clean-water rinse for 15 minutes.
   Scale is the leading cause of code 16 in hard-water areas — descale every 12-18 months.

3. Inspect the recirculation setup.

   If the system has a recirculation pump, confirm it stops when no hot water is being drawn.
   A pump running 24/7 keeps water in the heat exchanger spinning and slowly heating up — even in standby — until the high limit trips.

4. Test the thermistor.

   Disconnect the outgoing thermistor at the board.
   Measure resistance across the leads at room temperature — it should read in the kilo-ohm range that matches the Rinnai service spec for your model.
   Out of range means the sensor is reading false and needs replacement (15-40 USD).

5. Look for cross-connections.

   If a single-handle faucet or a recirculation valve is leaking, hot water can flow back into the cold inlet of the heater.
   The heater then tries to heat already-warm water and trips the high limit.
   Isolate fixtures one at a time to find the culprit.

6. Reset and observe.

   Press the controller off and on, or cycle the dedicated breaker for 30 seconds.
   Run hot water at a tub or shower (high-flow fixture) for 5 minutes.
   If code 16 returns, schedule service rather than continuing to reset.